EP1812551A2

EP1812551A2 - Method for removing bacteria from a fermentation broth

Info

Publication number: EP1812551A2
Application number: EP05798653A
Authority: EP
Inventors: Kevin O'mahony; Frank Hilbrig; Ruth Freitag; Ivo Schumacher
Original assignee: Dr Mueller AG; POLYTAG AG
Current assignee: Dr Mueller AG; POLYTAG AG
Priority date: 2004-11-15
Filing date: 2005-11-07
Publication date: 2007-08-01
Anticipated expiration: 2025-11-07
Also published as: EP1812551B1; WO2006050625A3; WO2006050625A2; US20100120127A1; ATE391171T1; DE502005003594D1

Abstract

The invention relates to method for the stepwise removal of bacteria from a fermentation broth. According to said method, the bacteria in the fermentation broth are mixed with a retention promoter mixture in a first process step and the pretreated bacteria are removed by precoat filtration in a second process step. The inventive method uses a retention promoter mixture from bentonite and polycations such as PEI or polyDADMAC and is suitable for treating large quantities of bacteria. The filter cake obtained with bentonite/PEI has a substantially improved quality.

Description

Process for the separation of bacteria from a fermentation broth

The present invention relates to a method for the stepwise separation of

Bacteria from a fermentation broth.

Precoat filtration is a well-known process for separating solids from liquids and gases. Special importance is attached to it in biotechnical processes, where cell material from a culture broth of a

Fermenters must be separated.

From GB 1, 082,862 a method for the separation of yeast cells by precoat filtration is known. It mentions filter aids such as starch and siliceous Fuller earths as well as Cellite. The behavior of the relatively large

Yeast cells are different from that of bacteria.

A Kerzenanschwemmfilter the type is known from EP 0,064,795. The known filter is used in the continuous separation of nonspecific sewage sludge. Filter aids are not mentioned.

From WO 01/81249 A1 a device for purifying water or gases of chemical or microbiological contaminants is known. As microorganisms bacteria or viruses are called. The known device consists of a container in which a retention agent bentonite, a

Polymer and other material may be included. Bentonite is known to fix bacteria, viruses and so on. Therefore, it is suggested that it can also remove bacteria from a ventilation system. The known method uses a polymer. The bentonite is at one

Material fixed in the container. There can be no question of a filtration with filter additives, ie a precoat filtration.

In the US 5,453,200 in the cultivation of microorganisms, these are isolated by filtration or centrifugation. After separation of the microorganisms, it is proposed to treat the supernatant filtrate with bentonite to absorb contaminating proteins. Bentonite is therefore not used in this case to improve the retention of microorganisms.

EP 0391 253 describes a process for the purification and deodorization of

Enzyme solutions, in particular the removal of disturbing colored and odoriferous admixtures with the aid of absorbents, including bentonite. Bentonite is not used to separate the production organisms.

The object of the invention is to provide a process which provides the efficient and complete removal of bacterial biomass from fermentations with the highest possible biomass concentrations. allows. It can be assumed that the isolated bacterial cells serve as base material for further processing.

The object is achieved according to the invention in that in a first process stage the bacteria-containing fermentation broth is mixed with a retention promoter and in a second process stage the pretreated bacteria are separated by precoat filtration. When

Retention promoters are mixtures of a polycation (poly (ethyleneimine) (PEI), poly (diallyldimethylammonium chloride) (polyDADMAC)) and a multi-layer silicate (bentonite) but also modified (positively charged) celluloses in question.

The concentration of biomass should preferably have an optical density OD ₆ oo of 15 at the time of precoat filtration. If necessary, this can be achieved by dilution. Be cultures with higher optical Densities without filtering prior to the filtration step, the efficiency of the cell retention decreases and the pressure drop across the filter cake increases sharply.

The precoat filtration is carried out at a flow rate of 500 to 1500 l / (m2 ^* h).

As a retention promoter mixture, the addition of 5 to 500 mg / l of polycation (PEI, polyDADMAC) and 2 to 10 g / l of bentonite has proven particularly advantageous. Under these conditions almost complete bacteria retention is guaranteed (at least> 98%). If smaller amounts of polycation and / or bentonite are added, the efficiency of bacterial retention decreases (<50%); Higher levels of polycations do not further improve filtration efficiency and result in higher filtration pressures, while at higher bentonite additions (greater than 10 g / L) unnecessarily bulky filter cakes and thus uneconomical filtration conditions are achieved. At the same time, filtration pressures also increase.

Bentonite is a multi-layered silicate, which is also known by the synonyms smectite and montmorillonite.

It has been found advantageous to use sodium activated bentonite, or sodium bentonite. Sodium activated bentonite has given particularly good results. Calcium bentonite has also proved useful.

As polycations for the retention promoter mixture are further

Poly (ethyleneimine) (PEI) from various manufacturers outstandingly suitable. PEI is a polycation composed of ethyleneimine units. High molecular weight PEI (1000 - 50000 g / mol) are better suited than low molecular weight (<1000 g / mol), with comparable molar mass, branched molecules are better suited than unbranched ones. A second polycation demonstrably good than

Retention promoter is suitable, poly (diallyldimethylammonium chloride) (polyDADMAC. During the first process step (treatment with the

Retention promoter mixture) is carried out under process conditions, it is advantageous that immediately before the filtration and before the addition of the filter aid, the pH of the suspension is increased to a value of 8, preferably by adding a suitable amount of Tris buffer.

Furthermore, it is advantageous for 100-1000 mM NaCl to be mixed in immediately before the filtration. The addition of Tris and NaCl has the advantage that the filtration pressure is reduced and thereby the permeability of the filter cake is slightly increased.

Bacteria are (in comparison to eukaryotic cells such as yeasts, fungi or animal / plant cells) very small organisms that are used in biotechnology in a variety of ways for production. Their complete separation from the culture supernatant by simple filtration without filter aid is impossible, the separation by precoat caused extremely voluminous, mechanically stable filter cake (storage of the bacteria in the cake), which would require in the case of the usual in bioproduction cell densities order of magnitude higher filter aid amounts than commonly considered to be industry-compatible. It is not possible the necessary amounts of filter aids by pretreatment of bacteria (such as polycations (eg PEI, PoIyDADMAC), multi-charged metal ions (eg Fe3 +, AI3 +, Ca2 +), charged adsorbents such as positively charged cellulose, cross-linking with negatively charged silicate particles) substantially to reduce. At the same time, in all cases, fixing the cells in the filter cake is insufficient for building a robust process. The bacteria are typically already affected by low pressure or

Volume flow fluctuations washed out of the filter cake again (bleeding of the bacteria). This makes it impossible to process the bacteria in the filter cake (washing, buffer exchange, cell lysis) and thus any process intensification approach through process integration.

The use of bentonite and polycations such as PEI or PoIyDADMAC as a retention promoter in precoat filtration surprisingly leads to a complete and completely stable separation of the bacteria in Filter cake manageable extent. The observed high efficiency of the

Bacterial retention has been shown to exceed a pure fixation effect by the retention promoter mixture and is likely due to the formation of a hydrogel by the bentonite during the incubation phase. In any case, bacterial filtration after treatment of the culture with PEI (polyDADMAC) / bentonite is of fundamentally different quality

(Pressure drop, filterability, bacterial retention, fixation of the bacteria in the filter cake) as an otherwise analogous filtration (filter aid, flow rate) without pretreatment or after pretreatment with a polycation alone.

Example of bacteria

Bacteria of the strain (E. coli DH5 from Clontech-BD Biosciences, Paolo Alto, California, USA) were cultured in a known manner in a bioreactor (Chemap FZ). The culture medium (pH 6.8) consisted of at least 10 g / l glucose, 2 g / l NaCl, 1.2 g / l MgSO4, 6 g / l Na2HPO4, 3 g / l KH2PO4, 3 g / l (NH4) 2SO4, 1.7 g / l citric acid, 0.01 g / l CaCl2 and trace elements (FeCl3 60 mg / l, Na2EDTA 8.4 mg / l, Zn (CH3COO) 2 8.0 mg / l, MnCl2 15.0 mg / l, CoCl2 2.5 mg / l, Na2MoO4 2.5 mg / l, CuCl 2 1.5 mg / l, H3BO 3 3.0 mg / l.). 02 was not measured, the

However, cultivation at maximum air flow, ie about 30 l / min performed. A temperature of 37 ⁰ C was set. After 16 hours the bioreactor was ready to harvest (OD600> 15).

The filtration was carried out by means of a laboratory candle filter (type TSD-0.012m2 of the

DrM, Dr. Müller AG, Männedorf, Switzerland). The laboratory candle filter has a surface area of 75 cm2 or 40 cm2 and allows, depending on the biomass concentration, the filtration of a few liters of cell suspension per batch. A membrane (B31M30, G11M200, or G11M080, all supplied by DrM) supported by a metal plate was used as a filter medium. Various filter aids (diatomaceous earths) have been used, including the company's "Celpure" materials, World Minerals Inc. (Advanced Minerals and Celites), St Barbara, CA, USA and the "Ce! Atom" materials of Eagle Picher Filtration and Minerals Europe.

The experiments were carried out under typical industrial conditions. The flow rate was set between 500 and 1500 l / (m2 * h). The amount of filter aid was between 10 and 20 g / l. All experiments were - unless otherwise indicated - carried out with bacterial cultures E. Coli the nominal OD600 of 15.

OD = optical density. The measurement is carried out in a known manner with light of wavelength 600 nm and indicates the turbidity of the solution. It gives the reading of the concentration of bacteria in the suspension.

Experiments with different types and grains as well as amounts of filter aids such as diatomaceous earths (diatomaceous earths) did not lead to success. Only uneconomically large amounts of kieselguhr brought a useful clarification. An effective and economical separation of the bacteria of a culture is only possible with filter aids. In addition, the bacteria are not sufficiently stable retained in the filter cake. This makes the filtration uncertain and impairs or makes impossible further treatment (washing, cell lysis) of the bacteria-containing filter cake.

Test results are shown in Tables 1 and 2:

Table 1 Results of the body feed filtration experiments with various diatomaceous earths as filter aids.

Amount and type of filter aid Final pressure Bacteria retention *

5 g / L Celpure P100 3 bar ** 72%

10 g / L Celpure P100 3 bar ** 65%

5 g / L Celpure P300 2.85 bar ** 36%

10 g / L Celpure P300 0.8 bar 38.5%

15 g / L Celpure P300 0.6 bar 50%

20 g / L Celpure P300 0.8 bar 74%

30 g / L Celpure P300 0.4 bar 75%

0.25 g / L Celpure P1000 1.8 bar 13%

1 g / L Celpure P1000 0.6 bar 16%

5 g / L Celpure P1000 O bar 19%

10 g / L Celatom FW12 0.9 bar 69% 30 g / L Celatom FP4 L35 66_%

due to clogging of the filter cake.

Table 2 Results of the filtration experiments with mixtures of fine and coarse diatomaceous earths as filter aids.

Ratio P1000: P100, total amount Final pressure Bacteria retention

P1000 alone (5 g / L) 0.05 bar 23%

9: l, 2 g / L 2.6 bar * 9%

9: I ₃ 5 g / L 1.1 bar 23%

9: 1, 10 g / L 0.3 bar 23%

9: 1, 15 g / L 0.2 bar 39%

3: 1, 2 g / l 2.5 bar * 19%

3: 1, 5 g / L 1.2 bar 27%

3: 1, 10 g / L 0.4 bar 33%

3: 1, 15 g / l 0.2 bar 43%

1: 1, 5 g / L 1.5 bar 53%

1: 1, 10 g / L 0.6 bar 37%

1: 4, 5 g / L 3.0 bar * 55%

1: 4, 10 g / L 1.5 bar 62%

Celatom FW6: FW12, 1: 1 10g / L 2.4 bar * 82%

* filtration had to be prematurely due to clogging Qf the filter cake.

Also experiments on filtration by means of other filter aids (cellulose, J. Rettenmaier & Söhne, Rosenberg, Germany, to observe any retention) or after flocculation of the bacteria by polycations (PEI,

PolyDADMAC) or multiply positively charged metal ions (Fe 3+, Al 3+, Ca 2+), Table 3, or adsorption on positively charged cellulose, Table 4, and cross-linking by positively charged ions (polycations, metal ions) with negatively charged filter aid particles, Table 5, It was to be noted, however, that the bacteria adsorbed on positively charged cellulose (also J. Rettenmaier & Söhne) (retention usually <50%) were unsuccessful in terms of a complete, but above all stable retention of the bacteria in a filter cake of industry-compatible dimensions. at the "industry-compatible" maximum filter aid amounts considered here), they were stably established in the filter cake after filtration and such filter cakes could be washed without noticeable bleeding of the bacteria with different wash buffers, for example, which has not been observed in any of the other experiments presented so far. Table 3 Results of the filtration experiments afiter flocculation with PEI. Conditions Final pressure Bacteria retention

FW 610 g / L, PEI 350 mg / L 1.6 bar 81%

FW 12 lOg / L, PEI 350mg / L 1.1 bar 77%

FWI 210g / L, PEI 325mg / L, pH 8, 50mM NaCl 3.0 bar 86%

FW1220g / L, PEI 325 mg / L, pH 8, 50 mM NaCl 1.7 bar 87%

FW1220g / L, PEI 337.5mg / L ₅ pH 8, 50mM NaCl 1.5bar 87.5%

FW1210g / L, PEI45mg / L 2.0 bar 87%

FW 1210 g / L, PEI425 mg / L • 1.4 bar 94%

FW 1210 g / L, PEI4150 mg / L 2.0 bar 100%

FW14 lOg / L, PEI 325 mg / L, pH 8, NaCl 50 mM 2.2 bar 83%

FWL 420g / L, PEI 337.5mg / L, pH 8 1.5 bar 87%

FW1420g / L, PEI 337.5mg / L, pH 8, NaCl 50mM 1.2 bar 86%

FW2010g / L, PEI 325 mg / L, pH _. 8, NaCl 50 mM 1.5 bar 74% FW2010g / L ₅ PEI450 mg / L 3.0 bar 70%

FW 2020 g / L, PEI 450 mg / L 2.6 bar 92%

* filtration had to be prematurely due to clogging of the filter cake.

Table 4 Capture / sedimentation of bacteria with charged cellulose.

Conditions BE600 / 20 BE600 / 20 BE600 / 20 FDY FIC EFC EFC

10 g / L 5g / L lg / L 600 200 900CT 950 C 10

10 g / L 10 g / L 10 g / L g / L pH 5.5

0.0 M NaCl 31% 27% 28% 23% 24%

0.1 M NaCl 23% 17% 12%

0.25 M 23.5% 18%. 11%

NaCl

0.5M NaCl 28% 22.5% 13% pH 7.0

0.0 M NaCl 29% 20% 27% 21% 18%

0.1 MNaCl 25% 22% 18%

0.25 M 30% 23% 16%

NaCl

0.5M NaCl 28% 17% 18% pH 8.0

0.0 M NaCl 29% 17% 24.5% 20% 17%

0.1 MNaCl 36% 34% 28%

0.25 M 38.5% 29% 27%

NaCl

0.5 M NaCl 37% 29% 28% Table 5 Filtration of bacteria cross linked to diatomaceous earth

Conditions Final pressure Bacteria retention

10g / L FW12, 10 mg / L AlCl ₃ 3.0 bar * 75.5% 10g / L FW12, 1 mg / L AlCl ₃ 3.5 74% 10 bar g / L FW12, 0.1 mg / L AlCl ₃ 2.0 bar 69%

10 g / L FW12 ₃ 5 mg / L PEI4 2.0 bar * 87%

10 g / L FW 12, 25 mg / L PEI4 1.4 bar * 94%

10 g / L FW 12, 150 mg / L PEI4 2.0 bar * 100%

10 g / L P1000, 75 mg / L PEI 4 1.1 bar 88%

10. g / L P1000, 100 mg / L PEI4 1.3 bar 97%

10 g / L P1000, 125 mg / L PEI4 ^' 1.2 bar 95%

10 g / L FW20, 50 mg / L PEI4 3.0 bar 70%

20 g / L FW 20, 50 mg / L PEI 4 2.6 bar 92%

10 g / L FW12, 10 g / L P1000, 5 mg / L PEI4 0.6 bar 75%

10 g / L FW12, 10 g / L P1000, 25 mg / L PEI4 0.4 bar 83%

10 g / L FW12, 10 g / L PlOOO, 150 mg / L PEI4. 1.5 bar 90.5%

* filtration had to be prematurely due to clogging of the filter cake.

Experiments with bentonite:

Bentonite is a multi-layer silicate that swells more or less strongly in contact with aqueous solutions. This swelling leads to absorption of various substances from the environment, including bacteria. After numerous preliminary experiments with sodium activated bentonite was added to the

Implementation of the embodiment selected. The mineral swells to an extent between sodium bentonite, which swells strongly, and calcium bentonite, which swells very little. The mineral has positive charges in the spaces of the intermediate layers. This allows an electrostatic interaction with the negatively charged bacteria. The mineral therefore theoretically has the ability to bind bacteria both by adsorption and by absorption.

The results of numerous attempts at precoat filtration of bentonite treated bacteria are summarized in Table 6. Both were

Attempts to pretreat the bacterial suspension with bentonite alone, as well as those in which in addition to bentonite repeatedly charged metal ions or polycations for pretreatment were used, considered during treatment with bentonite alone, especially in crops with low Biomass (OD600 <15) and minimal media (LB medium), which occasionally led to success, a robust and reliable process could only be established by the treatment (incubation) with the bentonite / polycation mixture.

Table 6, Filtration of bacteria Suspension in the presence of bentonite.

Conditions Final pressure Bacteria retention

20 g / L FW12, 5 g / L WGA, LB medium * 1. 4 bar 93%

20 g / L FW 14, 5 g / L WGA, LB medium * 3 bar -

20 g / L FW 14, 5 g / L WGA, LB medium * 1.3 bar -

20 g / L FW 14, 5 g / L WGA, LB medium, 250 raM 0.2 bar 52%

NaCl

20 g / L FW 14, 5 g / L WGA, LB medium, 500 mM 0.3 bar 53%

NaCl

20 g / L FW 14, 5 g / L WGA, LB medium, 50 mM 0.5 bar 55%

Tris pH 8, 250 mM NaCl

20 g / L FW 50, 5 g / L CV, LB medium * 1.3 bar 94%

20 g / L FW 14, 5 g / L WGA, 50 mM Tris pH 8, 250 0.4 bar 69% mM NaCl, 0.1 g / L FeCl ₃

20 g / L FW 14, 5 g / L WGA, 50 mM Tris pH 8, 250 1.4 bar 89% mM NaCl, 0.2 g / L FeCl ₃

20 g / L FW 14, 5 g / L WGA, 50 mM Tris pH 8, 250 1.8 bar 93% mM NaCl, 0.3 g / L FeCl ₃

20 g / L FW 14, 5 g / L WGA, 50 mM Tris pH 8, 250 2.2 bar 92% mM NaCl, 0.4 g / L FeCl ₃

20 g / L FW 14, 5 g / L WGA, 50 mM NaCl, 50 mg / L 0.4 bar 65%

PEI4

20 g / L FW 14, 5 g / L WGA, 50 mM Tris pH 8, 2.1 bar 100%

50mg / L PEI4

20 g / L FW 14, 5 g / L WGA, 50 mM Tris pH 8, 250 1.4 bar 100% mM NaCl, 50 mg / L PEI4

20 g / L FW 14, 5 g / L WGA, 50 mM Tris pH 8, 1000 0.6 bar 98% mM NaCl, 50 mg / L PEI4

20 g / L FW 14, 5 g / L WGA, 50 mM Tris pH 8, 250 0.6 bar 100% mM NaCl, 100 mg / L PEI4

20 g / L FW 14, 5 g / L WGA, 50 mM Tris pH 8, 250 0.2 bar 58% mM NaCl, 50 mg / L polyDADMAC

20 g / L FW 14, 5 g / L WGA, 50 mM Tris pH 8, 250 0.9 bar 100% mM NaCl, 150 mg / L polyDADMAC

* filtration terminated prematurely due to clogging of the filter cake, ** repetition of the experiment above with a culture containing much debris The filtration of the thus fixed bacteria surprisingly led to a nearly 100% retention. A washing out of the bacteria (recognizable by a clearly visible increase in the turbidity of the filtrate) is not observed even after the application of a pressure pulse in such filtrations.

The effectiveness of bentonite can not be explained by adsorption / absorption alone. There is a synergistic effect that obviously improves the retention of the bacteria. This effect is probably due to the formation of hydrogels by the bentonite. Best results are observed at pH 8, probably because here the positive charge of the PEI is strong enough for a stable interaction with the negatively charged bacteria. In experiments without addition of polycation, a reduction of the pH to values between 5.0 and 5.5 could be the method in the sense of a reduction of the otherwise comparable conditions needed

Improve filtration pressure. This is probably due to degradation of the pH-sensitive hydrogels. This also agrees with the observation that such suspensions must be further processed as quickly as possible after lowering the pH, otherwise both the bacteria ratio and the stability of the bacteria in the filter cake will decrease. Also one

Addition of up to 1 M NaCl has a decreasing effect on the pressure required for filtration, without noticeably reducing bacterial retention / stability.

Overall, these effects, however, especially in the filtrations without

To observe polycation additive. If the bacteria are filtered after pretreatment with bentonite / polycation at pH 8, the process is very robust and other parameters such as temperature, salinity or composition of the media barely play a role in the success. Embodiment:

At the end of the bacterial culture, the contents in the bioreactor (fermenter) are mixed with 5 g / l bentonite and 50 mg / l PEI and mixed at room temperature for 1 hour. Subsequently, the bacterial suspension is brought to a nominal biomass concentration of not more than OD600 = 15, a pH of 8, a NaCl concentration of 250 mM and a Tris concentration of 50 mM by adding a suitable Tris-buffered solution. Then the filter aid is added (e.g., 20 g / L Celatom FW 14) and the mixture is rapidly filtered at a flow rate of 600 L (m2 * H). Filtrations were carried out with and without the addition of bentonite. The mean values of the results are shown graphically. Show it:

Fig. 1 A comparison of the filtration with and without bentonite. Fig. 2 The retention capacity of the bacteria in the filter cake with and without

Bentonite.

In Figure 1, curve 1 shows the filtration without bentonite; Curve 2 shows the filtration with the addition of bentonite, wherein surprisingly a 95-100% retention of the bacteria, visible from a near-zero turbidity, is evident.

Filtration using bentonite gives a clear filtrate, depending on the type of bentonite used. To achieve the same effect with known filter aids alone would be required by an order of magnitude higher amounts of filter aid.

Figure 2 shows the results after a leaching of the filter cake. A washing buffer applied under high pressure to the filter cake without bentonite would lead to a detachment of the bacteria from the cake (left column). They are not tied to the cake. A filter cake with

Bentonite, on the other hand, can be washed without loss of bacteria (right column). As a result, the filter cake obtained with bentonite is of substantially better quality. A great advantage of the method according to the invention is that it is excellently suited for the treatment of large amounts of bacteria, since when using bentonite a permeable, stable filter cake is achieved. The filter cake can be easily washed with other liquids or subjected to further process steps such as in-situ cell lysis.

In addition, such a filter cake can also be easily dried by air. Further, at the end of filtration, the filter cake can be easily removed from the filter elements, such as a candle filter. There is no sticking to the filter media.

Claims

claims

1. A process for the stepwise separation of bacteria from a fermentation broth, characterized in that in a first

Process stage, the bacteria incubating the fermentation broth with a retention promoter mixture and separated in a second process stage, the pretreated bacteria by precoat filtration.

2. The method according to claim 1, characterized in that the precoat filtration is carried out at a flow rate of 500 to 1500 l / (m2 * h).

3. The method according to claim 1, characterized in that are used for the precoat between 2 and 20 g / l of filter aid.

4. The method according to claim 4, characterized in that the alluvium filtration with diatomaceous earths (diatomaceous earth) is carried out as a filter aid.

5. The method according to claim 1, characterized in that the

Fermentation broth is incubated for one hour with the retention promoter mixture.

6. The method according to claim 1, characterized in that 2 to 10 g / l of bentonite are used in the retention promoter mixture.

7. The method according to claim 8, characterized in that is used as bentonite sodium bentonite.

8. The method according to claim 8, characterized in that as bentonite

Calcium bentonite is used.

9. The method according to claim 1, characterized in that in the

Retention promoter mixture 0.5 to 500 mg / l PEI can be used.

10. The method according to claim 11, characterized in that is used in the retention promoter mixture PEI having a molecular weight between 1000 and 50,000 g / mol.

11. The method according to claim 13, characterized in that in the retention promoter mixture branched PEI is used.

12. The method according to claim 1, characterized in that in the

Retention promoter mixture 0.5 to 500 mg / l polyDADMAC can be used.

13. The method according to claim 1, characterized in that immediately before the filtration, the pH of the suspension is adjusted to the value 8.

14. The method according to claim 1, characterized in that immediately before the filtration 50 - 1000 mM NaCl are mixed.

15. The method according to claim 1, characterized in that immediately before the filtration, a buffer concentration of 50 mM is set.

16. The method according to claim 19, characterized in that immediately before the filtration, a buffer concentration of 50 mM Tris is set.

17. The method according to claim 1, characterized in that the concentration of bacteria in the fermentation broth has an optical density OD600 of at most 15.

18. The method according to claim 1, characterized in that the nominal

Concentration of the bacteria in the fermentation broth after incubation with the retention promoter and adjusted (diluted) to a value of OD600 = 15 immediately before filtration.

19. A method according to claim 22, characterized in that the nominal OD600 is adjusted together with the pH, the NaCl and the buffer concentration by a suitable Tris-buffered solution.

20. The method according to claim 1, characterized in that as

Retention promoter modified (positively-charged) celluloses can be used.